The present invention is directed to a tool bit with a substantially circular shank for insertion into a tool bit chuck of a hand-held tool for percussion drilling. The shank has at least two rotary entrainment or drive grooves whose side flanks, under load when drilling, are not diametrically opposite one another and the grooves are open on the end of the shank for receiving rotary entrainment members in the tool chuck. The shank is also provided with two locking grooves which are closed or have a reduced cross-section at the end adjacent to the end face of the shank for limiting axial movement of the tool bit. Further, at least one locking member of the tool chuck can be received in one of the locking grooves.
Tool bits of this type are disclosed in DE-PS 3716915 and D-PS 3941646. Such tool bits are designed for introduction into a tool chuck only in a very specific angular position.
It has been found to be disadvantageous in tool bits of this type that the space remaining between the rotary entrainment members for receiving the locking grooves is of different magnitudes. In the rough operation experienced at a construction site, it is possible that the rotary entrainment grooves are crushed to an extent after the tool bit has been used for a long time, that the remaining residual wall thickness between a rotary entrainment groove and the locking groove is reduced, whereby the stress peaks developed at this location result in fatigue fractures. Since such failure occurs within the tool chuck of the tool, damage results not only in the loss of the bit, but also the tool itself must be repaired at considerable expense by removing the shank end located in the tool. Due to the unequal design of a shank cross-section, the possibility of fracture can occur in new tool bits.